git-ls-files.txt: clarify -x/--exclude option

Since b5227d8, -x/--exclude does not apply to cached files.
This is easy to miss unless you read the discussion in the
EXCLUDE PATTERNS section. Clarify that the option applies
to untracked files and direct the reader to EXCLUDE PATTERNS.

Signed-off-by: Jay Soffian <jaysoffian@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-ls-files.txt

@@ -79,8 +79,9 @@ OPTIONS
 
 -x <pattern>::
 --exclude=<pattern>::
-	Skips files matching pattern.
-	Note that pattern is a shell wildcard pattern.
+	Skips untracked files matching pattern.
+	Note that pattern is a shell wildcard pattern. See EXCLUDE PATTERNS
+	below for more information.
 
 -X <file>::
 --exclude-from=<file>::